1. Field of the Invention
The invention relates to an acid nickel or nickel alloy electroplating bath and to a method for depositing a satin-finished nickel or nickel alloy coating.
2. Brief Description of the Related Art
Predominantly bright nickel or nickel alloy coatings which moreover should be well levelled are used in industry. However, it has been recognized early that satin-finished coatings may look esthetically and at the same time prevent from dazzling. When combined with semi-bright nickel coatings and with a chromium coating such coatings are just as corrosion preventing as a bright nickel coating. These satin-finished nickel coatings are therefore often used in automotive industry, in precision mechanics industry, for sanitary appliances as well as for furniture mountings.
Up to now nickel coatings can be produced with various methods:
It has been indicated in DE-OS 1 621 085 that the surface of the metal to be coated could first be roughened by means of sandblasting. Afterwards the surface would then be treated with a common electroplating bath to deposit a bright nickel layer. According to another method first a bright nickel coating could be provided with a mat finish by means of mechanical treatment. Due to this treatment, however, the corrosion resistance would be reduced considerably because the nickel layer would be weakened. It is further indicated that both methods described would suffer from being very complicated and expensive due to the mechanical treatment. Other methods are described in this document which allow deposition of satin-finished nickel coatings directly from the electroplating bath without any preceding or succeeding mechanical treatment.
For this purpose finely grinded material which is insoluble in these baths, such as for example kaolin, graphite, barium sulfate, glass, talcum powder, calcium oxalate and other substances, with a particle size of from 0.1 to 0.3 μm are added to the common nickel electroplating baths in considerable amounts. By intensively blowing air through the baths these substances are held in suspension and will be codeposited into the coating as nickel is deposited. It is indicated in this document that a certain roughness of the coating would emerge establishing satin-finished appearance. This method, however, would require a specific apparatus for carrying out the method, since the method could not be performed in conventional electroplating devices. For this reason additional costs would arise.
Because of the drawbacks of the conventional methods an acid nickel electroplating bath for the production of satin-finished nickel coatings is disclosed in DE-OS 1 621 985 as an improvement over the described methods in this document. For performing this method a bath is required that, additional to basic brightening compounds, contains substituted or unsubstituted ethylene oxide or propylene oxide or ethylene oxide propylene oxide adducts at a concentration of from 5 to 100 mg/l, these additional adducts being able to form a finely dispersed emulsion in the bath solution at a temperature of from 40 to 75° C.
Further an acid nickel, nickel/cobalt or nickel/iron electroplating bath are described in DE 25 22 130 B1, these baths being suitable for the deposition of satin-finished coatings. This bath contains liquid polysiloxane polyoxyalkylene block polymers in an emulsion in addition to primary and/or secondary brighteners.
Nickel coatings that are known from the disclosure in DE-OS 1 621 085 can be produced according to the method as described in DE-AS 1 621 087. Coatings exhibiting even satin-finish can be produced by cooling the bath liquid completely or partly below cloud temperature and subsequent heating the liquid to the working temperature again. Upon exceeding the cloud temperature nonionic surfactants precipitate due to the fact that the surfactants lose their hydrate sheath. The emulsified droplets formed are dissolved upon cooling the liquid and will once more be formed upon anewed heating. The nickel deposition is impaired selectively by precipitating droplets of the surfactant, without the droplets essentially being included into the nickel coating. The fact that much energy must be spent for heating and cooling the plating liquid as well as for pumping the liquid makes this method disadvantageous. Furthermore the maximum bath volume is limited to a certain value since the expenditure for heating and cooling the liquid and for pumping the liquid raises considerably if the bath volume exceeds 8.000 l. Under these conditions operation of the method is no longer economical. Moreover after a short time of carrying out this method lumps of the surfactants are formed in the bath solution which cause pores to be produced in the nickel coatings.
Due to the drawbacks mentioned above the method for producing semi-bright nickel or nickel/cobalt coatings as described in DE 23 27 881 A1 has been successful. In this method the mat coatings are generated by incorporating foreign matter into the coatings. The foreign matter is produced by bringing together cationic or amphoteric substances with organic anions. Quaternary ammonium compounds, derivatives of imidazolines, alkanolamine esters and surface active agents based on amino carboxylic acids are proposed in this document as cationic or amphoteric substances. By bringing together the cationic or amphoteric substances with the organic anions an emulsion is formed which together with basic brighteners being present in the nickel electroplating bath leading to a satin-finish by imparing nickel deposition.
Unfortunately this method also suffers from certain drawbacks: Within about three to five hours after making up the electroplating bath the surface of deposited nickel coatings becomes more and more rough. In part even visually detectable coarse nickel crystals appear on the surface which are not acceptable as to the appearance of the nickel surface. Therefore at least before eight hours of production have passed the bath liquid must be worked up by completely filtrating and cleaning it with filter material, such as for example cellulose, diatomaceous earth or even with activated carbon. The production break required for working the bath liquid up is especially very troublesome and expensive if a continuous plant is operated. Moreover a removable “silver layer” is generated if afterwards a chromium layer is deposited for 10 minutes or longer.
Several attempts have been made to get rid of the shortcomings mentioned. Therefore in DE 37 36 171 A1 a method for the deposition of satin-finished nickel coatings is described, the nickel bath liquid used for carrying out this method containing inter alia one or more basic brighteners, one or more anionic surfactants, one or more organic emulsion formers, one or more quaternary ammonium compounds as well as one or more acyclic or aromatic sulfinic acids. Preferably benzoic acid sulfimide, m-benzenedisulfonic acid, naphthalenetrisulfonic acid, diaryldisulfides, sulfonamides and N-sulfonyl carboxamides as well as the salts thereof being soluble in water are to be understood as basic brighteners. However, upon carrying out this method coatings with a constant appearance cannot be achieved without heating and cooling the bath liquid as before.
A further electroplating method for producing nickel coatings that have a non-dazzling appearance is disclosed in DE 195 40 011 A1. According to this document a nickel bath is used, that contains inter alia basic brighteners, organic sulfinic acids as well as surfactants. Additionally the bath contains substituted and/or unsubstituted ethylene oxide adducts or propylene oxide adducts or ethylene oxide propylene oxide adducts at such a low concentration that cloudiness is not visually detectable at the working temperature of the bath. The use of nonionic surfactants at the concentration indicated in this document does not guarantee, however, since their efficiency quickly diminishes and since the appearance of the coatings quickly changes.
Further an aqueous electroplating bath for depositing bright nickel or nickel/cobalt coatings is described in DE 21 34 457 C2. According to several examples in this document sulfosuccinic acid esters are added to the bath liquid which additionally contains saccharin as a secondary brightener. However, satin-finished nickel coatings were not produced with these baths. Furthermore a nickel bath for depositing satin-finished coatings is disclosed in Patent Abstracts of Japan, JP 56152988 A which contains surfactants selected from the group comprising alkyl aryl sulfonates and sulfosuccinic acid esters additional to saccharin as a brightener and ethylene oxide propylene oxide block polymer. In this case too it has be established that a satin-finished nickel coating could only be produced within a short period after the bath has been made up. After this period coatings were generated which exhibit a rough surface.
All methods described can only be carried out during a few hours. Within this period nickel coatings with more or less satisfactory satin-finish are obtainable. However, during this period of time roughness increases. After expiry of this period only rough nickel coatings can be deposited which are porous.
The problem of the present invention therefore consists in avoiding the disadvantages of the known electroplating baths and especially in finding an electroplating bath suitable for the production of a satin-finished nickel or nickel alloy coating and a method for producing satin-finished nickel coatings. When using this method it should be possible to generate nickel coatings with constant surface quality within a long period of time after the electroplating bath has been made up without the necessity to clean the bath liquid or work the bath up with any other means with excessive expenditure.